1. Field of the Invention
The present invention relates to a method of producing a printed wiring board and a copper foil for laser processing, particularly to a method of producing a printed wiring board which forms a via-hole for interlayer connection by a Cu direct method and a copper foil for laser processing.
2. Background Art
Conventionally, the multilayering of printed wiring boards has been spreading with the enhanced performance and downsizing in electronic devices and electric appliances. A multilayer printed wiring board is prepared by laminating three or more wiring layers through an insulating layer and the wiring layers are electrically connected by an interlayer connection means such as a via-hole and a through hole. A build-up process is known as a method of producing a multilayer printed wiring board. The build-up process is a production method sequentially laminating wiring layers on an inner layer circuit board through an insulating layer followed by interconnection of the wiring layers to finish a multilayered wiring board. For example, when an ultra-fine wiring pattern is formed by the modified semi-additive process (MSAP process) or the like, a build-up printed wiring board is produced by the following procedures. First, a core substrate provided with an inner layer circuit is laminated with copper foil through an insulating layer; a via-hole or the like is formed in the produced copper clad laminate by laser processing or the like; and electroless plating is carried out for interlayer connection. Next, a plating resist is provided on a seed layer (copper foil+electroless plating layer) depending on a wiring pattern; electroplating is carried out; and then the plating resist is removed and then the seed layer under the plating resist is removed by etching. The repeated above steps for a required number of times finishes a build-up multilayer printed wiring board having desired number of wiring layers.
According to finer wiring patterns, the interlayer connection performed by via-holes each having a top diameter of 100 micrometers or less has been increased in recent years. Such micro via-holes are generally drilled by laser processing using a carbon dioxide laser. On this occasion, a Cu direct method in which copper foil is directly irradiated with a carbon dioxide laser has been employed to drill the copper foil and an insulating layer at the same time. However, laser beam absorption of copper is very low in a wavelength range of far-infrared to infrared rays such as a carbon dioxide laser. So, when the micro via-holes are formed by the Cu direct method, pretreatment such as black-oxide treatment which increases absorption of the infrared laser-beam on the surface of a copper foil has been required.
However, when black-oxide treatment or the like is provided on the surface of a copper foil, the copper foil thickness decreases and deviates because the surface of the copper foil is etched. So, when a seed layer is removed, formation of a wiring pattern having high linearity and good line width is made difficult because the etching time should be set depending on the part of the seed layer having the largest thickness.
On the contrary, Japanese Patent Laid-Open No. 2001-226796 discloses a copper foil provided with an alloy layer mainly comprising Sn and Cu on the copper foil surface as a technology which does not require pretreatment before infrared laser processing. According to Japanese Patent Laid-Open No. 2001-226796, the infrared laser absorption of Sn is twice or more higher than that of Cu at the same room temperature and the same surface roughness. So, when an alloy layer mainly comprising Sn and Cu is provided on a copper foil surface, the copper foil surface is directly irradiated with the infrared laser beam without pretreatment such as black-oxide treatment to form a via-hole having a diameter of 100 μm.
Further, Japanese Patent Laid-Open No. 2001-308477 discloses a surface-treated copper foil for laser drilling in which a nickel layer or a cobalt layer having a specific thickness is provided on one side of the copper foil. By providing a nickel layer or a cobalt layer having a specific thickness on the surface of the copper foil, the temperature of the infrared laser-irradiated part is continuously held at a copper melting temperature or more, and it makes drilling of both the copper foil layer and the base material resin layer at the same time possible.
However, the copper foil for infrared laser drilling disclosed in Japanese Patent Laid-Open No. 2001-226796 employs a method in which a metallic Sn layer is provided on the surface of the copper foil by vapor deposition or plating, and then Cu—Sn alloy layer is formed on the surface of the copper foil by the diffusion through heat treatment. For this reason, the alloy layer has a distribution of Sn content along thickness direction, and the etching rate along thickness direction of the copper foil may deviates. Further, the outermost surface of the copper foil has an extremely high content of Sn. So, when a popular etchant for a copper foil is used, it is difficult to dissolve and remove the outermost surface by etching, and since the etching rate is uneven along thickness direction, thickness of the copper foil may deviate. Furthermore, the surface of the alloy layer may have an etching rate smaller than that of an electrolytic copper plated wiring pattern. So, a good wiring pattern is hardly obtained because the wiring pattern is more rapidly etched and a line width decreases when a seed layer is removed.
On the contrary, copper foil may be etched after removing only Sn by using an etchant selectively etches Sn. However, thickness of the copper foil after selectively etching Sn will have a deviation because the content of Sn in the alloy layer deviates as described above. So, formation of a wiring pattern having high linearity and good line width is made difficult because the etching time should be set depending on the part of the seed layer having the largest thickness also.
Further, when a multilayer printed wiring board is produced using the surface-treated copper foil disclosed in Japanese Patent Laid-Open No. 2001-308477, nickel layer or cobalt layer provided on the surface of the copper foil is selectively removed by etching to obtain a seed layer having a uniform thickness without a deviation in thickness in the copper foil. When the seed layer is removed by etching after the electrolytic copper plated wiring pattern is formed, only the copper foil having a uniform thickness may be dissolved, and a good wiring pattern having a fine line width might be obtained. When this surface-treated copper foil is used, pretreatment such as black-oxide treatment to the copper foil surface is not required in the production of a printed wiring board, but a step of selectively removing the nickel layer or cobalt layer by etching is indispensable after infrared laser processing. So, it is impossible to reduce process steps.
So, an object of the present invention is to provide a method of producing a printed wiring board which reduces process steps, is excellent in infrared laser processability and is suitable for formation of an excellent wiring pattern; and to provide a copper foil for laser processing and a copper-clad laminate.